Method of controlling well casing corrosion



United States Patent Ofilice 2,847,070 Patented Aug. 12, 1958 METHOD OF CONTROLLING WELL CASING CORROSION No Drawing. Application July 5, 1955 Serial No. 520,154

5 Claims. (Cl. 166-4) This invention relates to an improved method or inhibiting corrosion of oil or gas well casings.

In the drilling of oil and gas wells by the rotary method, the drilling mud used to remove cuttings, re tain the desired pressure in the well bore, etc., is ordi narily a fresh water base mud. Upon completion of the drilling operation, the drill string is removed from the well bore, leaving the bore at least partially filled with the drilling mud. The well casing is then lowered through the drilling mud to the desired depth, and cement is pumped down through the casing and then up the annulus between the casing and the walls of the Well bore to set the casing in the well. The cement only partially covers the outside of the casing, thereby leaving the fresh water base mud filling the annulus from the top of the cement to the surface of the well.

The drilling mud has a substantially different salt concentration than the salt concentration of the formation Waters, thereby inducing electrochemical E. M. F.s between the mud column and the formations which are referred to as spontaneous potentials or SPs. The casing offers a low resistance to the induced current, thereby causing the flow of an electrical current from the formation through the drilling mud, the casing, and then back through the drilling mud to another portion of the formation. These currents are termed SP currents. At the points where the current leaves the casing, concentrated corrosion will take place as the iron of the casing goes into solution. This corrosion has been particularly noted where a fresh water mud column contacts a formation containing connate water of a different salinity. The E. M. F.s and resulting currents originate as a combination of diffusion potentials and ion exchange membrane potentials. Generally speaking, the currents will be large in cases where clay-free sandstones or limestones are in contact with dense purely argillaceous shales.

The present invention contemplates a novel method of protecting casings from corrosion resulting from SP currents, wherein the drilling mud filling the annulus between the casing and the walls of the well bore from the cement to the surface of the well is given a salinity corresponding to the salinity of the formation waters. Thus, the electrochemical E. M. F.s between the mud and the formation waters will be reduced to substantially zero and the destructive currents will be eliminated.

An important object of this invention is to extend the service life of oil and gas well casings, and particularly casings extending through formations having fluids of high salinities.

Another object of this invention is to materially reduce corrosion of casing extending through a well bore.

A further object of this invention is to surround an oil or gas well casing with a drilling mud having an ionic activity substantially equal to the ionic activity of the formation fluids surrounding the well bore and the casing.

Another object of this invention is to provide an economical method of inhibiting corrosion of oil and gas Well casing.

Other objects and advantages of the invention will be apparent as the description proceeds.

Broadly stated, the present invention may be defined as a method of inhibiting corrosion of a casing extending through a well bore traversing a plurality of formations, which comprises filling the annular space between the casing and the walls of the well bore with a composition opposite each formation having sub stantially the same ionic activity as the formation fluid of the respective formation.

Theinitial step in preventing corrosion by the present method is to determine the ionic activity of the various formation fluids surrounding the well bore, before the casing is set. Inasmuch as the ionic activity is principally controllcd by the salinity of a formation fluid, the ionic activity may be determined from formation water samples, salinity sampling of cores obtained during the drilling operation, or in any other desired water sampling procedure. In many instances, however, the usual sampling is either unavailable or impractical. In this event, use is made of an electric log run in the well bore while the original drilling mud stands in the well, and before the casing is set.

An SP (spontaneous potential) curve is obtained by an electric log and is indicative of the spontaneous potentials existing opposite the various formations. The salinities of the formation lluids are correlatable with the SP curve by the following relation:

SP K log 2 wherein SP is the spontaneous potential (in millivolts) of a formation obtained by electric logging with reference to the potentials opposite dense shales; R is the resistivity (in ohm-meters) of the drilling mud filtrate; R is the resistivity (in ohm-meters) of the formation fluid; and K is a constant. K will vary with the type of formation. For example, in a clear sandstone at approximately room temperature, K will equal approximately 70, and shaly sandstones under the same conditions will have a K from 40 to Also, K is directly proportional to the absolute temperature of the formation. In any specific example, K will either be known or can be readily determined by testing core samples, as is well known in the art of electric logging. The resistivities, R and R,,, are in turn inversely proportional to the salinity of the mud filtrate and formation fluid, respectively. Therefore, when the SP and resistivity of the drilling mud are known, the salinity of the specific formation fluid can be calculated.

The original drilling mud is then replaced by a composition having an ionic activity substantially equal to the ionic activity of the formation fluids to prevent a contrast between the various formation fluids and the composition immediately surrounding a casing in the well. It is preferred that the original drilling mud standing in the well be conditioned and utilized for this purpose. The conditioning of the drilling mud may be performed in any desired manner, such as by the addition of water-soluble inorganic salts and bases, whereby the ionic activity of the mud is matched with the ionic activity of any desired formation water. In addition, however, the condition-ed mud must also have a pH above 7 and preferably a pH of 8 or 9.

A preferred method of conditioning the mud is by recirculation through the drill pipe before the casing is landed. However, the conditioned drilling mud may be injected into the well in any of several other ways, de-

pending upon local conditions and available equipment. For example, the casing may be lowered in the well bore and the conditioned mud pumped down the casing preceding the cement used in setting the casing. in this procedure, the conditioned mud will be forced upwardly through the annulus between the casing and the walls of the well bore to displace the original drilling mud stand ing in the well. Thus, the conditioned mud will completely surround the casing from the top of the cement to the surface of the well, or to any desired height.

Another injection procedure is to set the casing in the well and perforate the casing above the cement plug. A separate tubing may then be run into the casing and packed off above and below the perforations. A suitable port collar is disposed on the tubing between the packers and opposite the perforations, and the lower end of the tubing is capped. The conditioned mud may then be pumped down the tubing where it will be discharged out wardly through the port collar and perforations into the annulus between the casing and the walls of the well bore. As in the previous example, the conditioned mud will then be forced upwardly in the annulus to replace the original drilling mud. Subsequently, the perforations in the casing may be closed by squeeze cementing, as is well known in the art.

Still another method of replacing the original drilling mud with the conditioned mud, is to proceed from the top of the well. When the zone of drilling mud to be replaced is positioned above a formation having higher fluid loss characteristics, the conditioned mud may be forced down the annulus around the casing to the desired position.- The original drilling mud preceding the conditioned mud will be forced out into a formation below the desired replacement zone, thereby making way for the conditioned mud. An analogous procedure, commonly known as Bradenhead cementing, is used in cementing-off specific formations, and is well known in the art.

It will be apparent that the ionic content of the conditioned mud may be varied to provide a composition surrounding the casing which has an ionic activity substantially equal to the ionic activity of the adjacent formation fluid. Thus, no appreciable contrast will exist in the salinity of the conditioned drilling mud and formation fluids, whereby the electrical potentials in the well bore are substantially reduced to zero and SP current corrosion of the casing will be inhibited. Generally speaking, the ionic activity of the conditioned mud is controlled only in the zones which have produced marked SPs by the electric log. Marked SPs may be defined as variations of ten or more millivolts in the SP curve. The remaining formations do not provide a substantial contrast in salinity with the original drilling mud and, therefore, no appreciable currents will originate in these zones.

It is also to be noted that the ionic content of the conditioned mud should not be rapidly changed to provide adjacent contacting zones of mud having contrasting ionic concentrations; otherwise the conditioned mud would provide difiusion potentials giving rise to new spontaneous currents which would cause corrosion of the casing. Therefore, if zones of greatly different connate water salinities are encountered in the well, the complementary zones of conditioned mud must either be separated by non-porous partitions, such as packers, or the salinity of the conditioned mud should be adjusted to approximately the average of the formation fluid salinities. In the usual case, when the salinity of the conditioned mud is approximately matched with the average of differing connate water salinities, the salinity contrast between the conditioned mud and the various formation waters will be substantially reduced and will not give rise to any serious spontaneous currents.

From the foregoing it will be apparent that the present invention provides a novel method of extending the servieelife of oil and gas well casings. The casingis sun- 4 rounded by a composition which reduces the electrical potentials in the well bore to substantially zero, to in turn inhibit corrosion of the casing caused by SP currents. Also, the composition will inhibit acidic corrosion of the casing. A convenient composition is made by adjusting the ionic content of the drilling mud, whereby an economical and easily handled composition is obtained. Each formation fluid, and particularly the formation waters giving rise to marked SP potentials, are opposed by the conditioned drilling mud having an ionic activity substantially equal to the specific formation water.

While particular embodiments of the invention have been described, it will be understood, of course, that the invention is not limited thereto since many modifications may be made, and it is, therefore, contemplated to cover by the appended claims any such modifications as fall within the true spirit and scope of the invention.

The, invention having thus been described, what is claimed and desired to be secured by Letters Patent is:

1. A method of inhibiting corrosion of a casing extending through a well bore wherein the well bore traverses a plurality of formations, which comprises running a log of said well bore to obtain the ionic activity of the water in each of the formations, and filling the annular space between the casing and the walls of the well bore opposite each of said formations with a drilling mud having an ionic activity corresponding substantially to the ionic activity of the water of each of said formations as obtained by said log.

2. A method of inhibiting corrosion of a casing extending through a well bore wherein the well bore traverses a plurality of formations, which comprises running an electric log in the well to obtain the ionic activity of the fluids of the various formations, and filling the annular space between the casing and the walls of the well bore opposite each of said formations with a composition having an ionic activity corresponding substantially to the ionic activity of the water of each of said formations as obtained by said electric log.

3. A method of inhibiting corrosion of a casing extending through a well bore wherein the well bore traverses a plurality of formations, which comprises running an electric log in the well to obtain the ionic activity of the various formation waters from the resulting spontaneous potential in millivolts curve of the electric log, and filling the annular space between the casing and the Walls-of the well bore opposite each of the formations with -a drilling mud having an ionic activity substantially equal to the ionic activity of the water of each of said formations as obtained by said electric log.

4. A method of completing a well traversing a plurality of formations, which comprises running an electric log in the well while the original drilling mud stands inthe well, to obtain the ionic activity of the waters of the various formations from the resulting spontaneous potential in millivolts curve of the electric log, setting a casing in the well, and displacing the remaining drilling mud in the annulus between the casing and the walls of the well bore opposite the formations exhibiting marked spontaneous potentials in millivolts with a composition having an ionic activity corresponding substantially to the ionic activity of each of said formations as obtained by said electric log.

5. A method of completing a well traversing a plurality of formations, which comprises running an electric log in the well while the original drilling mud stands in the well, to obtain the ionic activity of the waters of the various formations which produce marked spontaneous potentials in millivolts on the spontaneous potential in millivolts curve of the electric log, setting a casing in the well, and displacing the original drilling mud remaining in the annulus between the casing and the walls of the well'bore opposite each of said formations producing marked spontaneous potentials in millivolts with a conditioned drilling mud having an ionic activity correspond- 5 6 ing substantially to the ionic activity of each of said forma- 2,625,374 Neuman Ian. 13, 1953 tions as obtained by said electric log. 2,764,24 Rohfback e a1 P 25, 1956 References Cited in the file of this patent OTHER RLbERENcES 5 Mills: Protection of Oil and Field Equipment Against UNITED STATES PATENTS Corrosion, Bulletin 233, Department of Interior, GPO 2,429,593 Case Oct. 28, 1947 1925, 77 to 79. 

1. A METHOD OF INHIBITING CORROSION OF A CASING EXTENDING THROUGH A WELL BORE WHEREIN THE WELL BORE TRAVERSES A PLURALITY OF FORMATIONS, WHICH COMPRISES RUNNING A LOG OF SAID WELL BORE TO OBTAIN THE IONIC ACTIVITY OF THE WATER IN EACH OF THE FORMATIONS, AND FILLING THE ANNULAR SPACE BETWEEN THE CASING AND THE WALLS OF THE WELL BORE OPPOSITE EACH OF SAID FORMATIONS WITH A DRILLING MUD HAVING AN IONIC ACTIVITY CORRESPONDING SUBSTANTIALLY TO THE IONIC ACTIVITY OF THE WATER OF EACH OF SAID FORMATIONS AS OBTAINED BY SAID LOG. 